Splitting hybrids

Until fuel-cells become commonplace, our choking planet is offered some relief by the hybrid vehicle.

Until fuel-cells become commonplace, our choking planet is offered some relief by the hybrid vehicle, a sort of half-way-house that uses a combination of fossil and electric power to eke out an extra 20 – 30 miles per gallon.

Honda and Toyota have already introduced hybrids and others are working on them. But Demaco, a UK company, has developed a hybrid concept with a difference.

In general, the 2 power sources in hybrids are combined either in parallel or series. In parallel, a fuel tank supplies petrol to the IC engine and batteries supply power to an electric motor. Both the engine and electric motor can turn the transmission at the same time. In a series hybrid, the IC engine is not connected to the wheels, but charges a battery. Thus, the battery powers the vehicle.

Both have their merits. Parallel hybrids allow smaller engines to be used, whilst series hybrids, since the engine runs at a constant speed, can operate at a high efficiency level.

Demaco claims that its ‘Split Hybrid’ concept combines the best of both these systems.

Brainchild of designer Peter Jelf, and currently under evaluation by a major truck manufacturer, the split hybrid uses a turbo charged diesel engine. Its output shaft is clutched to the through shaft of an electric motor but directly linked to a 3-phase generator (which continuously charges the batteries). Via an intelligent control unit and inverter they drive the electric motor, which is connected to the wheel axle.

When speed requirement is low or power demand is high, the IC engine clutch is disengaged, leaving the vehicle driven solely by electric power. As demand for power reduces, the control unit matches the IC engine revs to the motor shaft and engages the clutch. At this point the electric motor shaft is purely a lay shaft between the IC engine and the wheel axle. A control unit selects or combines the various operating modes to ensure optimal efficiency.

Toyota’s Prius also switches between series and parallel operation, but Demaco’s Rod Dawson believes that the Split Hybrid is superior.

Firstly, the split hybrid uses a smaller engine than the Prius (900 cc compared to1500 cc.) This, says Dawson, ‘leads us to believe that the Prius uses the IC engine through a wider range of speeds resulting in the less than impressive mpg returns and lower efficiency.’ The Split Hybrid has the engine running at 2 speeds, one to run ancillaries via the generator when the motor is driving the wheels and the other at around 90% of max efficiency speed during which time it can maintain a cruise at 55 – 65 mph. All extra power is provided by the electric motor.

Demaco’s method of overcoming the power split problem is also more practical argues Peter Jelf. The Split Hybrid uses a clutch to disengage the main drive elements while the Prius uses planetary gears. ‘This system and the fact that up to one third of the power available from the IC engine is constantly applied to the generator explains the relatively poor fuel economy evident with the Prius’ says Jelf.